Dual function heat withdrawal in a fluidized catalytic cracking-regeneration process

ABSTRACT

A process and apparatus for cooling FCC catalyst that provides two modes of operation for a remote heat exchanger type cooler. In the first mode the exchanger is operated in a backmix fashion where catalyst is circulated between the cooler and a disengagement zone. Regenerated catalyst is withdrawn from an outlet located close to the heat exchanger inlet so that essentially all of the heat removed by the cooler serves to reduce the temperature of catalyst entering the reaction zone. This mode of operation is particularly useful in obtaining a benefit from the cooler when processing light to moderate FCC feeds. In the second mode, the exchanger is operated in a flow through mode where hot catalyst is withdrawn from the disengaging zone cooled in the exchanger and passed into the combustion zone. The second mode of operation is used to withdraw heat from the overall regeneration process in the case of heavy FCC feedstock conversion.

BACKGROUND OF THE INVENTION

The field of art to which this invention pertains is fluid particlecooling. It particularly relates to the combustion of combustiblematerial from a particulated solid such as fluidizable catalyst whichhas been at least partially deactivated by the deposition thereon of acombustible material, such as coke. The present invention will be mostuseful in a process for regenerating coke-contaminated fluid crackingcatalyst, but it should find use in any process in which combustiblematerial is burned from solid, fluidizable particles.

DESCRIPTION OF THE PRIOR ART

The fluid catalyst cracking process (hereinafter FCC) has beenextensively relied upon for the conversion of starting materials, suchas vacuum gas oils, and other relatively heavy oils, into lighter andmore valuable products. FCC involves the contact in a reaction zone ofthe starting material, whether it be vacuum gas oil or another oil, witha finely divided, or particulated, solid, catalytic material whichbehaves as a fluid when mixed with a gas or vapor. This materialpossesses the ability to catalyze the cracking reaction, and in soacting it is surface-deposited with coke, a by-product of the crackingreaction. Coke is comprised of hydrogen, carbon and other material suchas sulfur, and it interferes with the catalytic activity of FCCcatalysts. Facilities for the removal of coke from FCC catalyst,so-called regeneration facilities or regenerators, are ordinarilyprovided within an FCC unit. Coke-contaminated catalyst enters theregenerator and is contacted with an oxygen containing gas at conditionssuch that the coke is oxidized and a considerable amount of heat isreleased. A portion of this heat escapes the regenerator with the fluegas, comprised of excess regeneration gas and the gaseous products ofcoke oxidation. The balance of the heat leaves the regenerator with theregenerated, or relatively coke free, catalyst.

The fluidized catalyst is continuously circulated from the reaction zoneto the regeneration zone and then again to the reaction zone. The fluidcatalyst, as well as providing catalytic action, acts as a vehicle forthe transfer of heat from zone to zone. Catalyst exiting the reactionzone is spoken of as being "spent", that is partially deactivated by thedeposition of coke upon the catalyst. Catalyst from which coke has beensubstantially removed is spoken of as "regenerated catalyst".

The rate of conversion of the feedstock within the reaction zone iscontrolled by regulation of the temperature, activity of catalyst andquantity of catalyst (i.e. catalyst to oil ratio) therein. The mostcommon method of regulating the reaction temperature is by regulatingthe rate of circulation of catalyst from the regeneration zone to thereaction zone which simultaneously increases the catalyst/oil ratio.That is to say, if it is desired to increase the conversion rate, anincrease in the rate of flow of circulating fluid catalyst from theregenerator to the reactor is effected. Inasmuch as the temperaturewithin the regeneration zone under normal operations is considerablyhigher than the temperature within the reaction zone, this increase ininflux of catalyst from the hotter regeneration zone to the coolerreaction zone effects an increase in reaction zone temperature.

Previous politico-economic restraints which were put upon thetraditional lines of supply of crude oil made necessary the use, asstarting materials in FCC units, heavier-than-normal oils. Thus, greatemphasis was put on the ability of FCC units to cope with feedstockssuch as residual oils and possibly mixtures of heavy oils with coal orshale derived feeds.

Recent downward pricing of crude oil supplies have, at leasttemporarily, blunted the drive for processing increasingly heavier feedsin FCC units. Instead the prevailing uncertainty in available feedstockcomposition makes it advantageous to have an FCC unit that can handle awide variety of feeds.

The chemical nature and molecular structure of the feed to the FCC unitwill affect that level of coke on spent catalyst. Generally speaking,the higher the molecular weight, the higher the Conradson carbon, thehigher the heptane insolubles, and the higher the carbon to hydrogenratio, the higher will be the coke level on the spent catalyst. Also,high levels of combined nitrogen, such as found in shale derived oils,will also increase the coke level on spent catalyst. The processing ofheavier and heavier feedstocks, and particularly the processing ofdeasphalted oils, or direct processing of atmospheric bottoms from acrude unit, commonly referred to as reduced crude, does cause anincrease in all or some of these factors and does therefore cause anincrease in coke level on spent catalyst.

This increase in coke on spent catalyst results in a larger amount ofcoke burned in the regenerator per pound of catalyst circulated. Heat isremoved from the regenerator in conventional FCC units in the flue gasand principally in the hot regenerated catalyst stream. An increase inthe level of coke on spent catalyst will increase the temperaturedifference between the reactor and the regenerator, and in theregenerated catalyst temperature. A reduction in the amount of catalystcirculated is therefore necessary in order to maintain the same reactortemperature. However, this lower catalyst circulation rate required bythe higher temperature difference between the reactor and theregenerator will result in a fall in conversion, making it necessary tooperate with a higher reactor temperature in order to maintainconversion at the desired level. This will cause a change in yieldstructure due to an increase in thermal versus catalytic selectivitywhich may or may not be desirable, depending on what products arerequired from the process. Also there are limitations to thetemperatures that can be tolerated by FCC catalyst without there being asubstantial detrimental effect on catalyst activity. Generally, withcommonly available modern FCC catalyst, temperatures of regeneratedcatalyst are usually maintained below 1400° F., since loss of activitywould be very severe at about 1400°-1450° F. If a relatively commonreduced crude such as that derived from Light Arabian crude oil werecharged to a conventional FCC unit, and operated at a temperaturerequired for high conversion to lighter products, i.e. similar to thatfor a gas oil charge, the regenerator temperature would operate in therange of 1600°-1800° F. This would be too high a temperature for thecatalyst, require very expensive materials of construction, and give anextremely low catalyst circulation rate. It is therefore accepted thatwhen materials are processed that would give excessive regeneratortemperatures, a means must be provided for removing heat from theregenerator, which enables a lower regenerator temperature, and a lowertemperature difference between the reactor and the regenerator.

The prior art is replete with disclosures of FCC processes which utilizedense or dilute phase regenerated fluid catalyst heat removal zones orheat exchangers that are remote from and external to the regeneratorvessel to cool hot regenerated catalyst for return to the regenerator.Examples of such disclosures are as set forth in Daviduk et al.4,238,631; Harper U.S. Pat. No. 2,970,117; Owens U.S. Pat. No.2,873,175; McKinney U.S. Pat. No. 2,862,798; Watson et al. U.S. Pat. No.2,596,748; Jahnig et al. U.S. Pat. No. 2,515,156; Berger U.S. Pat. No.2,492,948; Watson U.S. Pat. No. 2,506,123; Lomas U.S. Pat. No.4,396,531; Lomas et al. U.S. Pat. No. 4,353,812; and Lomas et al. U.S.Pat. No. 4,439,533. At least one of the above U.S. patents (Harper)discloses that the rate of return of the cooled catalyst to theregenerator may be controlled by the regenerator (dense catalyst phase)temperature.

An important consideration in the above FCC processes involvingregenerator heat removal is the method of control of the quantity ofheat removed. In Harper U.S. Pat. No. 2,970,117 and Huff U.S. Pat. No.2,463,623, the sole method involves regulation of the rate of flow ofregenerated catalyst through external catalyst coolers. This method ofheat removal, utilizing external coolers and varying the rate ofcatalyst circulation through them as the exclusive means of control ofthe heat exchanger duty, involves the continual substantial changing ofthe catalyst loading on the regenerator with the associated difficultyor impossibility of maintaining convenient steady state operations. Animproved method of remote cooler, heat removal is disclosed in Lomas etal. U.S. Pat. No. 4,353,812 where the heat transfer coefficient acrossthe heat transfer surface is controlled by varying the catalyst densitythrough regulation of fluidizing gas addition. The principle ofcontrolling heat removal with fluidizing gas addition is used in LomasU.S. Pat. No. 4,439,533 to operate a backmixed cooling zone wherecatalyst to be cooled circulates in and out of a cooler inlet openingwithout a net transport of catalyst through the cooler. One method ofcontrol that has been purposefully avoided in the operation of most heatremoval zones is the circulation rate of cooling medium. Consequently,in order to prevent overheating and possible failure of the coolingtubes, cooling medium circulates through the tubes at a high andconstant rate.

Although the various cooler designs will remove heat from the FCCprocess, none of these cooler designs provide the flexibility foradvantageous use in processing a wide range of FCC feeds. The presentinvention allows an external type heat removal zone to be usedadvantageously for processing light to heavy FCC feedstocks.

SUMMARY OF THE INVENTION

Accordingly, the invention is, in one embodiment, a process for thecooling of hot fluidized solid particles contained in a first densephase fluidized bed of the particles. The hot particles are circulatedfrom the first bed through a cooler inlet to a cooling zone separatefrom the first bed and in open communication therewith. As long as heatremoval requirements remain relatively low, the cooling zone operates ina backmix mode wherein the hot particles are continuously backmixedbetween the cooling zone and the first bed and heat is withdrawn fromthe hot particles by indirect heat exchange with a cooling fluidenclosed in a heat exchange means inserted into the cooling zone toproduce relatively cool particles. The particles are maintained in thecooling zone as a second dense phase fluidized bed by passing afluidizing gas upwardly through the second bed. The first and secondbeds comprise a continuum throughout which the particles arecontinuously circulated. Fluidized solid particles are removed from thefirst bed for use outside the process through a withdrawal pointcomprising an upper outlet located proximate the cooler inlet. In thisway, a blend of particles having a temperature intermediate thetemperature of the uncooled particles in the first bed and cooledparticles in the cooling zone are removed from the first bed. When heatremoval requirements for the first bed increase, the cooling zone isoperated in a flow through mode wherein hot particles are transportedfrom the first bed through the cooler inlet and passed downwardlythrough the cooling zone while yet maintaining a second dense phase inthe cooling zone. Passing the particles downwardly through the coolingzone increases the total particle circulation rate, and can therebyincrease the heat withdrawal from the cooling zone. Downward movementalso draws particles away from the cooler inlet and hinders circulationof particles from the cooler inlet to the withdrawal point. Accordingly,the invention lies, at least in part, in providing a cooling zone havingone mode for primarily cooling particles removed from the first bed anda second mode for cooling particles removed from the bottom of thecooling zone.

Whether operating in a flow or backmix mode, the quantity of heatremoved is influenced by the heat transfer coefficient between thecatalyst and the heat exchange means, and the differential temperaturebetween the catalyst and the cooling fluid. The heat transfercoefficient rises as the quantity of fluidizing medium increases. Thus,heat removal is controlled, at least in part, by varying the quantity offluidizing gas. The differential temperature is a function of catalystcirculation through the cooler. Consequently, variation in the catalystcirculation offers another method of controlling heat removal which canbe utilized most directly in the flow through mode of cooler operation.

In a second embodiment, the invention is an apparatus for cooling hotfluidized particles, which apparatus comprises in combination: (a) hotparticle collection chamber; (b) a shell and tube heat exchanger ofvertical orientation, remote from the collection chamber, having theshell closed at the bottom and having the upper portion of the shell ofthe heat exchanger in sealed communication with an opening in thecollection chamber such that particles can circulate to and from thecollection chamber through the shell; (c) an outlet at a bottom portionof the shell of the heat exchanger for removing particles from theexchanger through a conduit connected thereto; (d) a valve forregulating flow through the conduit such that interrupting flow throughthe conduit allows operation of the exchanger in a complete backmix modeand opening the valve changes operation of the cooler to an at leastpartial flow through mode; (e) means for injecting and distributingfluidizing gas at a bottom portion of the shell side of the heatexchanger, such that fluidizing gas can pass into the shell side andmaintain a continuously fluidized particle bed therein and regulatingthe flow of fluidizing gas into the heat exchanger thereby regulatingthe heat transfer coefficient between the outside surface of the tubesof the heat exchanger and the fluidized particle bed, and therebyregulating the duty; (f) inlet and outlet conduits connected to thetubes of the heat exchanger, such that a cooling fluid can flow throughthe tubes; (g) a withdrawal point comprising an outlet in the collectionchamber, proximate the heat exchanger opening, for removing particlesfrom the collection chamber.

A highly preferred embodiment of this invention uses the cooling processof this invention in the regeneration of catalyst particles for an FCCoperation. In this embodiment, an oxygen containing regeneration gas andcoke-contaminated fluidized catalyst enter the lower locus of acombustion zone maintained at a sufficient temperature for cokeoxidation. Hot flue gas from the combustion zone transports hotregenerated catalyst into a disengaging zone wherein the flue gas isseparated and withdrawn. Hot catalyst collects at the bottom as afluidized bed in the bottom of the disengaging zone. The disengagingzone has a withdrawal point at its lower locus for removing catalystfrom the bed for use in an FCC reaction zone. A cooling zone, of thetype previously described, has an inlet in communication with thefluidized bed and located proximate the withdrawal point. This coolingzone may again be operated, according to this invention, in either thebackmix mode or flow through mode. The ability to operate in either theflow through or backmix mode allows burn kinetics to be maintained andcooler utilization to continue when processing light or heavyfeedstocks. When processing a heavy feed in the FCC reaction zone, therelatively large amount of coke produced will require high heatwithdrawal and an overall reduction of catalyst temperatures throughoutthe regenerated zone. In this situation, the cooler operates in a flowthrough mode to maximize heat withdrawal and circulate cooled catalystto the combustion zone so that overall catalyst temperatures are keptbelow about 1300° F. Thus, in the heavy feed processing case, catalystwithdrawn from the disengaging zone has about the same temperature ascatalyst in the regenerator. If a relatively light FCC feedstock becomesavailable, a relatively lower amount of coke is produced in theregeneration zone and the cooler is not needed to keep catalysttemperatures throughout the regeneration below 1300° F. However, incertain circumstances, it would be advantageous to reduce thetemperature of catalyst entering the reaction zone from the catalystwithdrawal point--temperatures on the order of 1250° F. beingparticularly preferred. Using the cooler in a flow through mode toreduce the temperature of the catalyst at the withdrawal point wouldlower catalyst temperatures throughout the regeneration zone therebyinterfering with the burn kinetics for coke combustion which favor ahigher temperature. However, using the cooler in the backmix mode onlyreduces catalyst temperatures near the catalyst withdrawal point so thatoverall the regenerator still operates at temperatures favorable forburn kinetics and coke combustion while relatively cooler catalyst iswithdrawn for use in the reaction zone. Although a single functioncooler, receiving catalyst from the withdrawal point and changing cooledcatalyst to the reaction zone, could be used to lower the temperature ofcatalyst entering the reaction zone, such a cooler would depress thecatalyst temperature too severely when called on to meet the heatremoval demands of heavy feed processing. In addition, such a singlefunction cooler could not be isolated from catalyst flow in the event ofmechanical failure.

Other embodiments of the present invention encompass further detailssuch as process streams and the function and arrangement of variouscomponents of the apparatus, all of which are hereinafter disclosed inthe following discussion of each of these facets of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional, elevation view of a regeneration apparatusaccording to one embodiment of the present invention, showing acombustion zone 10, a disengagement zone 20, and a cooling zone (heatexchanger) 30.

FIG. 2 is a plan view of the regeneration apparatus taken along line2--2.

The above-described drawing is intended to be schematically illustrativeof the present invention and not be a limitation thereon.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, in its process aspects, consists of steps for thecooling of a fluidized particulate solid. An important application ofthe invention will be for a process for the combustion of a combustiblematerial from fluidized solid particles containing the combustiblematerial, including the step of introducing oxygen containing combustiongas and the fluidized solid particles into a combustion zone maintainedat a temperature sufficient for oxidation of the combustible material.The combustible material will be oxidized therein to produce the firstdense phase fluidized bed of hot fluidized solid particles cooled by theprocess of the invention.

The above combustion zone may be in dilute phase with the hot particlestransported to a disengaging zone wherein the hot particles arecollected and maintained as the first bed, or the combustion zone may bein dense phase and in itself comprise the first bed.

In a particularly important embodiment of the invention, there will beincluded steps for the regenerative combustion within a combustion zoneof a coke containing FCC catalyst from a reaction zone to form hot fluegas and hot regenerated catalyst, disengagement and collection of thehot regenerated catalyst, cooling of the hot regenerated catalyst in aheat removal or cooling zone, and the use of at least a portion of thecooled regenerated catalyst for control of the temperatures of thecatalyst returning to the reaction zone. As used herein, the term "hotregenerated catalyst" means regenerated catalyst at the temperatureleaving the combustion zone, from about 1300° to about 1400° F., whilethe term "cool regenerated catalyst" means regenerated catalyst at thetemperature leaving the cooling zone, the latter of which is up to 200°F. less than the temperature of the hot regenerated catalyst. When heatremoval requirements from the regenerative combustion stage are low, aswill occur when processing more traditional FCC feeds such as vacuum gasoils, the cooling zone is operated in a backmix zone wherein the hotregenerated catalyst is continuously circulated through the combustionzone with no net downward movement of catalyst through the cooling zone.As the coke making tendencies of the feedstock increase with a shifttoward processing the previously mentioned heavier than normal oils, thecooling zone is operated in a flow through mode so that cool catalystpasses directly into the combustion zone to effect an overalltemperature reduction throughout the combustion zone and disengagementzone. In the backmix mode there will be a temperature gradient at thebottom of the disengagement zone, with the coolest catalyst beingproximate to the opening to the cooling zone and the hottest catalystbeing at the portion of the bottom of the disengagement zone furthestfrom such opening. By removing catalyst for the reaction zone from thedisengagement zone at a point proximate the cooler opening, essentiallyall of the heat removed from the cooler lowers only the temperature ofcatalyst returning to the reaction zone.

Reference will now be made to the attached drawings for a discussion ofexamples of the regeneration process embodiment and associated apparatusof the invention. In FIG. 1, regeneration gas, which may be air oranother oxygen-containing gas, enters the combustion zone 10 through aline 11, and is distributed by a dome style distribution grid 12. Airleaving the grid mixes with coke contaminated catalyst entering thecombustion zone through a conduit 13. These streams are shown as flowingseparately into the combustor zone 10, however each stream could flowtogether into a mixing conduit before entering combustion zone 10. Cokecontaminated catalyst commonly contains from about 0.1 to about 5 wt. %carbon, as coke. Coke is predominantly comprised of carbon, however, itcan contain from about 5 to about 15 wt. % hydrogen, as well as sulfurand other materials. The regeneration gas and entrained catalyst flowsupward from the lower part of combustion zone 10 to the upper partthereof in dilute phase. The term "dilute phase", as used herein, shallmean a catalyst/gas mixture of less than 30 lbs/ft³, and "dense phase"shall mean such mixture equal to or more than 30 lbs/ft³. Dilute phaseconditions, that is, a catalyst/gas mixture of less than 30 lbs/ft³, andtypically 2-10 lbs/ft³, are the most efficient for coke oxidation. Asthe catalyst/gas mixture ascends within combustion zone 10, the heat ofcombustion of coke is liberated and absorbed by the now relativelycarbon-free catalyst, in other words by the regenerated catalyst.

The rising catalyst/gas stream flows through riser conduit 14 andimpinges upon the top of lateral conduit 15, which impingement changesthe direction of flow of the stream and directs the catalyst and gasmixture through outlets 16. The impingement of the catalyst/gas streamupon surface 15 and the change of direction through outlets 16 causesalmost all of the hot regenerated catalyst flowing from the combustionzone to disengage from the flue gas and fall to the bottom portion ofdisengagement zone 20 which comprises a hot particle collection chamberor fluid particle collection section. Although zone 20 is referred to asa disengaging zone, this term also embraces the possibility thatadditional regeneration or combustion may be carried out in this zone.The catalyst collection area of the disengagement zone may be an annularreceptacle, as shown, or any other shape appropriate for collectingcatalyst particles. Catalyst in the bottom of the collection zone ismaintained as a dense fluidized bed 26 having an upper lever 27. Thegaseous products of coke oxidation and excess regeneration gas, or fluegas, and a small uncollected portion of hot regenerated catalyst flow upthrough disengagement zone 20 and enter catalyst/gas separators such ascyclones 21 through an inlet 22. Catalyst separated from the flue gasfalls from the cyclones to the bottom of disengagement zone 20 throughdip legs 23 and 24. The flue gas exits disengagement zone 20 via conduit25, through which it may proceed to associated energy recovery systems.

With further reference to FIG. 1, the cooling zone is comprised of aheat exchanger 30 having a vertical orientation with the catalyst in theshell side and the heat exchange medium, supplied by lines 32 and 32',passing through a tube bundle 31. The preferred heat exchange mediumwould be water, which, in further preference, would change onlypartially from liquid to gas phase (steam) when passing through thetubes. It is also preferable to operate the heat exchanger so that theexchange medium is circulated through the tubes at a constant rate. Thetube bundle in the heat exchanger will preferably be of the "bayonet"type wherein one end of the bundle is unattached, thereby minimizingproblems due to the expansion and contraction of the heat exchangercomponents when exposed to and cooled from the high regenerated catalysttemperatures. The heat transfer that occurs is, from the catalyst,through the tube walls, and into the heat transfer medium. The top ofthe shell is in sealed communication with the bottom portion of thedisengagement zone through a conduit portion 34 and an inlet 35 whichserves as a withdrawal point. Fluidizing gas, preferably air, is passedinto a lower portion of the shell side of heat exchanger 30 via line 36,thereby maintaining a dense phase fluidized catalyst bed in the shellside. The line 36 has valve 36' positioned across line 36 regulates theflow of fluidizing gas. The fluidizing gas effects turbulent backmixingand flow to and from the disengagement zone when the exchanger is in abackmix mode and allows catalyst transport when the exchanger is in aflow through mode. The level of the dense phase catalyst bed in thedisengagement zone will be kept above the opening into the shell so thatthe catalyst, when operating in the backmix mode, may freely backmix andcirculate throughout the inside of the shell and the bottom of thedisengagement zone.

When operating in the flow through mode, cool catalyst is withdrawn froma lower portion of exchanger 30 and returned to the combustion zone 10.Catalyst is withdrawn from a lower portion of the cooling through aconduit 37 having a flow control valve 38. Valve 38 regulates catalystflow out of conduit 37 and is fully closed when operating the exchangerin a complete backmixed mode. Upon opening valve 38, catalyst flows intoan external riser 39. A fluidizing gas, preferably a small portion ofthe combustion gas entering the combustion zone via line 11, enters thebottom of riser 39 through pipe 40 and lifts catalyst from the bottom ofriser 39 and transports the cool catalyst into combustion zone 10through riser outlet 41.

FIG. 1 shows a preferred embodiment of heat exchanger 30 and the mannerof the interconnection of heat exchanger 30 with disengagement zone 20.Heat exchanger 30 is shown with the shell side completely filled with adense phase fluidized catalyst bed which extends up to level 27, level27 being well above the connection between the heat exchanger anddisengagement zone. Catalyst freely circulates and backmixes throughoutthe heat exchanger shell and disengagement zone forming a dense phasecontinuum. Fluidizing gas, such as air, which enters the shell via line36 (air may be introduced at one or more points in the shell in additionto that shown) rises upward and flows into the disengagement zone whereit ultimately leaves the system with the flue gases.

The tube bundle shown is of the aforementioned bayonet type in which thetubes are attached at the bottom or "head" of the heat exchanger, butnot at any other location. A typical configuration of tubes in thebayonet-type bundle would be one inch tubes each ascending from inletmanifold 42 in the head up into the shell through a three inch tubesealed at its top, each one inch tube emptying into the three inch tubesin which it is contained just below the sealed end of the three inchtube. A liquid, such as water, would be passed up into the one inchtubes, would empty into the three inch tubes, would adsorb heat from thehot catalyst through the wall of the three inch tubes as it passeddownward through the annular space of the three inch tubes and wouldexit the heat exchanger, at least partially vaporized, from outletmanifold 43 in the head. It is essential that the quantity of hotparticles or catalyst which enter heat exchanger 30 be sufficient tomaintain a depth of dense phase fluid catalyst bed which substantiallysubmerges the tubes in the dense phase bed.

A conduit removes regenerated catalyst from the disengaging zone anddelivers it to an FCC reaction zone (not shown). The flow of hotcatalyst into the disengagement zone will always exceed the hot catalystexit flow requirements via conduit 44. At least a portion of catalystnot exiting via conduit 44 will be circulated to the combustion zone.Shown in FIG. 1 is an external conduit 45 and control valve 46 throughwhich the catalyst may pass from the disengagement zone to thecombustion zone.

Although FIG. 1 illustrates a single heat exchanger with associatedcirculating catalyst conduit, it should be understood that otherconfigurations are possible, such as two heat exchangers, of the designillustrated, side by side with the conduit 42 between them.

The flow through mode of the exchanger is used to transfer cooledcatalyst from the cooling zone to combustion zone. Cooled catalystentering the combustion zone effects an overall temperature reductionthroughout the combustion and disengagement zone. Therefore in the flowthrough mode a relatively uniform temperature exists in thedisengagement zone and the temperature of the catalyst withdrawn fromconduit 44 will approach the maximum combustion or regeneration zonetemperature. Operational methods and benefits for a flow through typecooler, especially in processing heavy FCC feedstocks, are well knownand can be found in the previously cited background patents, inparticular U.S. Pat. No. 4,434,245. A disadvantage of this mode ofoperation is that the temperature of the catalyst leaving conduit 44cannot be reduced without depressing temperatures throughout thecombustion and disengagement zones.

The backmix mode of cooling zone operation as practiced in thisinvention primarily reduces the temperature of catalyst removed from thedisengaging zone by conduit 44. It is known that backmixing can beobtained within the heat exchanger at reasonable superficial gasvelocities to circulate catalyst between the cooling zone anddisengaging zone. The air affects the heat transfer coefficient directlyby affecting the superficial velocity over the heat exchanger tubes andindirectly by influencing the extent of mass flow of catalyst from thedisengagement zone through the heat exchanger. The higher mass flow willalso result in a higher heat exchanger duty because the average catalysttemperature in the heat exchanger will be higher thereby providing ahigher temperature difference (ΔT) to which the amount of heat transferis directly proportional. Additional details on the operation of abackmix cooling zone can be found in U.S. Pat. No. 4,439,533. Relativelycool catalyst, circulated about the inlet of the cooling zone migratesto the outlet of conduit 44. Locating the cooling zone inlet 35 and theoutlet 44' of conduit 44 in close proximity brings about this migration.FIG. 2 depicts the relative location of outlet 44' and inlet 35.Preferably, the outlet and inlet will be within a 120° sector of thedisengaging periphery. Thus, as conduit 44 removes catalyst from thedisengaging, it draws catalyst away from the opening 35 and encouragesthe flow of hot catalyst particles across the catalyst opening 35. Theoutlet for conduit 45 has a location generally opposite the outlet 44'and inlet 35 in order to prevent the migration of cooled catalyst towardconduit 45 and ultimately into the combustion zone. Normally the backmixmode of cooling will be practiced until temperatures in the regenerationsection begin to exceed 1300° F.

FIG. 2 also shows a preferred orientation for outlet openings 16 where,in plan, the openings lie between the outlet of conduit 45, andinlet-outlet pair 35 and 44. Due to the disengaging effect of lateralconduit 15, most of the hot catalyst transferred to the disengaging zonefrom the combustion zone will enter regions of bed 27 which are remotefrom inlet 35, outlet 44' and any region therebetween. Therefore withthis arrangement, a substantial portion of the catalyst removed byconduit 44 must first flow past cooling zone inlet 35, therebyfacilitating the withdrawal of relatively cooler catalyst from opening44'. This arrangement also allows catalyst entering the disengagementzone to enter conduit 45 without flowing past opening 35 so that cooledcatalyst does not enter the combustor through conduit 45. Where catalystis dropped into the disengaging zone by a device having more than twooutlets, the outlets should be located to minimize hot catalyst additionto the region of catalyst bed 26 that supplies catalyst to outlet 44'and inlet 35.

In a highly preferred embodiment, the cooling zone operates in a backmixmode when an FCC reaction zone, associated therewith, processes a lightto moderately heavy FCC feed. Significant yield advantages for selectedproducts can be obtained by lowering the temperature of catalyst andraising the temperature of the feed entering the reaction zone. Ashereinbefore described, backmix operation of the cooling zone willselectively reduce the temperature of removed catalyst. This reductionoccurs without a direct affect on overall regenerator temperatures. Ofcourse, considered alone, heat removal from the catalyst entering thereaction zone would ultimately lead to lower overall temperatures in thecombustion and disengaging zones. However, in this preferred form ofoperation, additional heat associated with the higher temperature feedoffset the reduced heat input from the catalyst so that the temperatureregime of the combustion and disengaging zones remains unaffected. Thus,favorable reaction kinetics are maintained in the combustion zone by acooling zone that also has the capability to effect an overall reductionof combustion and disengaging zone temperatures when desired.

The following example demonstrates the advantages of using the coolingzone to reduce the temperature of catalyst entering the reaction zonewhen processing a moderately heavy FCC feed. The feed in this example isa blend of vacuum gas oil and residual oil having the properties setforth in Table 1.

                  TABLE 1                                                         ______________________________________                                        API                        23.8                                               S Wt. %                    1.38                                               RAMS BOTTOM CARBON Wt. %   1.9                                                Ni/V Wt. - PPM             1/4                                                VOL - % 1050° F.    18                                                 ______________________________________                                    

EXAMPLE I

In this example the FCC feed was processed in an FCC reactor-regeneratorhaving an FCC riser reaction zone at process conditions summarized inTable 2. This example did not include the use of a cooler, therefore,the no entry is shown across from the cooler duty item. Yield resultsfor this operation are also summarized in Table 2.

                  TABLE 2                                                         ______________________________________                                        PROCESS APPLICATION FEEDSTOCK-                                                PRODUCT FLEXIBILITY                                                                                 EXAM-    EXAM-                                          PROCESS CONDITIONS    PLE I    PLE II                                         ______________________________________                                        CATALYST TO COMBUSTOR TEMP                                                                          980      980                                            °F.                                                                    CATALYST TO REACTOR TEMP °F.                                                                 1396     1355                                           CAT/OIL               BASE     BASE                                           FEED TEMP. °F. 350      510                                            COOLER DUTY BTU/LB COKE                                                                             --       1940                                           MAT ACTIVITY          BASE     BASE + 3                                       CAT ADDN #/BBL        0.27     0.27                                           YIELDS                                                                        C.sub.2 -Wt. %        5.1      4.3                                            C.sub.3 LV. %         10.1     10.4                                           C.sub.4 LV. %         13.5     13.9                                           C.sub.5 -GASOLINE LV. %                                                                             52.9     54.9                                           LCO LV. %             16.5     15.9                                           CO LV. %              11.8     11.0                                           COKE Wt. %            5.5      5.5                                            TOTAL LV. %           104.7    106.6                                          ______________________________________                                    

EXAMPLE II

In this example, the same feed was processed in an FCC reactorregenerator identical in all respects to that of Example I except forthe addition of a cooling zone arranged in accordance with thisinvention. Process conditions and yield results for this example areagain summarized in Table 2. A comparison of the conditions shows thatthe only major differences, apart from the cooler duty, were, forExample II, a lower temperature for the catalyst entering the riser anda higher temperature for the feed to the riser.

The yield results for the two examples demonstrate the bebefits of thecooler operation, at constant coke production, in the increased liquidvolume yields of gasoline which accompanied an overall gain in totalliquid volume yields. Thus, the gain in liquid volume yields is madewith a selectivity toward gasoline.

It should be emphasized, however, that the FCC embodiment illustrated bythe Figures and the examples is only one possible application of thepresent invention which in its broadest sense is a process for coolingany hot fluidized particles for any purpose. The apparatus aspect of thepresent invention in its broadest sense, as summarized above, may alsobe identified in the Figures. Thus, the bottom of disengagement zone 20comprises the hot particle collection chamber or fluid particlecollection section. Heat exchanger 30 is the shell and tube heatexchanger of vertical orientation. Conduit 37, its attachment toexchanger 30 and riser 39 provide the outlet and conduit for removingparticles from the exchanger. Valve 38 provides the valve for regulatingflow through the conduit line 36. Lines 32 and 33 are the cooling fluidinlet and outlet conduits.

What is claimed is:
 1. A method of cooling fluidized cracking catalystin a catalyst regeneration zone for use in a catalytic cracking reactionzone that allows cooling of the catalyst for the reaction zone with andwithout overall cooling of the catalyst throughout the regenerationzone, said process comprising:(a) introducing oxygen containingregeneration gas and coke contaminated fluidized catalyst into a lowerlocus of a combustion zone maintained at a temperature sufficient forcoke oxidation and therein oxidizing coke to produce hot regeneratedcatalyst and hot flue gas; (b) transporting said hot flue gas and saidhot regenerated catalyst from an upper locus of said combustion zoneinto a regenerated catalyst disengaging zone, wherein said hotregenerated catalyst is separated from said flue gas; (c) withdrawingregenerated catalyst from a withdrawal point at a lower locus of saiddisengaging zone and transporting said regenerated catalyst to saidfluidized catalytic cracking reaction zone; (d) circulating catalystfrom said disengaging zone into a cooling zone having an inlet locatedin a 120° sector of a horizontal plane passing through said inlet andwithdrawal point, said sector being taken about the centerline of saiddisengaging zone and containing said inlet and said withdrawal point;(e) removing heat from said catalyst in said cooling zone by indirectheat exchange with a cooling fluid enclosed in a heat exchange meansinserted into said cooling zone to produce relatively cool regeneratedcatalyst; (f) passing a fluidizing gas upwardly through said coolingzone in sufficient quantity to maintain a dense fluidized bed in saidzone; (g) operating said cooling zone for a limited time withoutdownward flow through the cooling zone, to directly exchange catalyst insaid cooler with catalyst in said disengaging zone through the inlet ofsaid cooling zone and withdraw relatively cool catalyst from saidwithdrawal point without directly effecting substantial catalyst coolingthroughout said regeneration zone; and (h) withdrawing cooled catalystfrom a lower locus of said cooling zone and transporting said cooledcatalyst into said combustion zone to directly effect catalyst coolingthroughout said regeneration zone.
 2. The process of claim 1 whereinsaid cooling fluid is circulated through said heat exchange means at aconstant rate.
 3. The process of claim 1 wherein said cooling fluidcomprises water.
 4. The process of claim 1 wherein the quantity of saidfluidizing gas is adjusted to maintain exchange of catalyst in saidcooling zone with catalyst in said disengaging zone when catalyst iswithdrawn from the lower locus of said disengaging zone.
 5. The processof claim 1 wherein said cooled catalyst is transported upwardly intosaid combustion zone by a lift gas.
 6. The process of claim 1 whereincatalyst from said combustion zone is downwardly discharged into thedisengaging zone through a multiplicity of outlets and each outletdischarges the catalyst along a trajectory lying outside a sector of thedischarge zone containing the inlet of said cooling zone and saidwithdrawal point.